Fire tubes usually have finning arranged on the inner surface of the tube, in order to increase the heat exchange efficiency.
Such fire tubes can be used in different types of plants such as for example heat exchangers or boilers.
Hot fumes produced by combustion pass through a fire tube. The combustion can also occur in the first length of the fire tube. The fire tube should internally withstand temperatures ranging from 1200° C. in the inlet section (base) to 40° C. in the outlet section (head), where the fumes have cooled.
In the absorption air-conditioning systems such fire tubes are used to release heat to an admixture of water and ammonia, in order to separate ammonia from water, thus obtaining ammonia vapor at the head and a liquid solution poor in ammonia at the base.
In such plants a fire tube is subjected to high external pressure, usually up to 35 bars, that tends to make it collapse. The standards set out that the fire tube in such plants should withstand an external pressure of four times the design pressure. For this reason, the side wall of the fire tube must have thickness adequate to withstand the difference of pressure between the outside and inside.
Also the difference of temperature among the various parts of the fire tube causes different thermal expansion and consequent structural stress or thermal stress.
Fire tubes must therefore ensure good heat exchange coefficient, high resistance to temperatures, high structural resistance to external pressure and thermal stresses.
Different fire tubes are known in the field of art.
In US 2010/0307729 A1 a fire tube is described, having longitudinal fins arranged on the inner surface. Such fins are made with U-shaped profiles for an easier connection to the inner surface of the tube. Furthermore, the height of the fins increases moving closer to the outlet section and are arranged on rings.
In U.S. Pat. No. 5,913,289 a fire tube is described having a corrugated metal sheet applied on the inner surface, in order to improve the heat exchange. Such a metal sheet is however subjected to significant thermal stress.
In U.S. Pat. No. 6,675,746 B2 the fins on the inner surface of the fire tube are formed by pins, in order to improve the resistance to thermal stress.
A problem with the fire tubes is the reduced efficacy of the heat exchange.
Another problem is related to pressure drop, specifically due to the narrowing of the passage section, such as for example in US 2010/0307729 A1 when fumes pass from a fin ring to the following one.
Another problem is to obtain uniform temperature distribution, or a desired temperature profile, on the outer surface of the fire tube.
Another problem is to improve the structural resistance of the tube against both the pressure difference between outside and inside, and thermal stresses.